An electronic device may be fabricated from one or more conventional semiconductor materials, such as silicon (Si), germanium (Ge) and gallium arsenide (GaAs) and include regions of N- and P-type conductivity which are tailored for particular device applications. Thus, selection of a semiconductor material is based not only on the material's inherent characteristics such as: bandgap, band type (direct or indirect), carrier mobility and effective mass, dielectric constant and lattice constant and structure, but also based on the type and concentration of the impurity (donor or acceptor) used in the device's intentionally doped active region(s).
Accordingly, control of a semiconductor material's characteristics can be achieved by including selected amounts and types of electrically active dopants to form N- and P-type regions of varying electrical conductivity and carrier mobility. The inclusion of dopants into semiconductor materials can be achieved using conventional processing techniques, such as in-situ doping, deposition, diffusion and ion-implantation. Many semiconductor devices include various intentionally doped N- and P-type regions to cause the semiconductor material to have uniform characteristics over a relatively wide temperature range.
The electrical behavior of a semiconductor depends primarily on the number of free charge carriers (electrons or holes) and the associated carrier mobility. In particular, conventional doped semiconductors, such as N-type silicon, exhibit three distinct regions of operation. These regions are graphically illustrated in FIG. 1, where the free electron carrier concentration (n) is plotted as a function of inverse temperature. These regions are the ionization, extrinsic and intrinsic regions, as shown. At very low temperatures near absolute zero (0 K), N- or P-type dopants are not electrically active to contribute to the semiconductor's conductivity. Electrical activation involves the giving up an electron by a donor (N-type) impurity, or the capture of an electron from the valence band by an acceptor (P-type) impurity. However, as the temperature is increased, a percentage of the dopants are activated through thermal ionization, thereby yielding free carriers and increasing the material's conductivity. In the ionization region, the free carrier concentration increases exponentially as a function of temperature.
At higher temperatures, the thermal energy eventually becomes sufficient to fully ionize the intentional dopants. This region, which occurs when the thermal energy exceeds the donor activation energy for N-type material, or exceeds the acceptor activation energy for P-type material, is referred to as the extrinsic region of operation. As illustrated, the free carrier concentration (n) in a conventional doped semiconductor such as Ge, Si or GaAs is substantially uniform throughout the entire extrinsic region. The donor and acceptor activation energies are functions of the depth of the corresponding intentional impurity in the bandgap, either with respect to the conduction band edge (E.sub.c) or valence band edge (E.sub.v), respectively. Donor and acceptor activation energies may also be functionally dependent on the concentrations of the corresponding intentional dopant impurities, through an inverse relationship.
At even higher temperatures, the thermal energy approaches that required to generate intrinsic carriers in substantially greater concentration than the already fully activated intentional impurities. This is an intrinsic property of a conventional semiconductor and which depends on the magnitude of the bandgap energy (E.sub.g). The bandgaps for Ge, Si and GaAs are 0.66 eV, 1.1 eV and 1.42 eV, respectively.
Once the intrinsic carrier density from thermal generation exceeds the concentration due to the intentional dopant, the semiconductor exhibits intrinsic behavior dominated by the intrinsic conductivity of the semiconductor. This is shown graphically in FIG. 2 for both lightly and highly doped N-type semiconductors (N.sub.D1 and N.sub.D2 respectively). For N- and P-type conventional semiconductors having non-negligible degrees of compensation, the intrinsic carrier density n.sub.i or p.sub.i exceeds N.sub.d -N.sub.a or N.sub.a -N.sub.d, respectively. The conductivity of an N- or P-type semiconductor rises abruptly at T.sub.f, which is the freeze-out temperature (i.e., the temperature at which the thermal energy is insufficient to activate the dopant impurities).
The intrinsic carrier concentration for Ge, Si and GaAs as a function of temperature is graphically illustrated in FIG. 3. As shown, these semiconductors exhibit extrinsic behavior at room temperature (300 K) when intentionally doped to a concentration greater than 1.times.10.sup.14 cm.sup.-3. Because conventional semiconductors such as Ge, Si and GaAs already exhibit extrinsic behavior at room temperature, attempts to extend the extrinsic region of operation have typically been directed towards extending the high temperature end of the extrinsic region. Such attempts typically include the steps of intentionally doping the semiconductor regions of an electronic device at relatively high levels so that the intrinsic carrier concentration will not exceed the intentional dopant concentration in the expected temperature range of operation.
Such steps are typically not necessary for relatively wide bandgap materials where E.sub.g is greater than 1.5 eV, because semiconductors having large E.sub.g typically do not experience intrinsic operation, except at extremely high temperatures on the order of 1000 K or higher and these temperatures typically exceed the thermal breakdown limits of the electronic device's other materials such as packaging, wiring and contacts and passivation. For example, a wide bandgap material such as diamond exhibits a negligible intrinsic carrier concentration of less than 1.times.10.sup.10 cm.sup.-3 at 1000 K.
Accordingly, for conventional semiconductors such as Si, Ge and GaAs, which can be doped with a wide variety of shallow donor and acceptor impurities having low activation energies, extrinsic operation over a temperature range extending to room temperature and even below can be readily achieved. However, correspondingly low temperature limits of extrinsic operation for wide bandgap semiconductors having relatively deep donor and acceptor impurities typically cannot be achieved. Typical wide bandgap semiconductors include: (i) diamond (C); (ii) IV-IV carbides (SiC and Si.sub.x Ge.sub.y C.sub.1-x-y); (iii) III-V nitrides and phosphides (cubic BN, CN, GaN, BP, GaAs.sub.1-x P.sub.x (x=0.1.fwdarw.1) and Al.sub.x Ga.sub.1-x N.sub.y P.sub.1-y); and (iv) II-VI selenides, tellurides, oxides and sulfides (CdSe, CdS, ZnS, Zn.sub.x Cd.sub.1-x S.sub.y Se.sub.1-y, CdTe and ZnO).
Moreover, typical dopant impurities in diamond, SiC and GaN typically exhibit relatively large acceptor ionization energies of 0.35-0.37 eV, 0.2-0.7 eV and 0.16-0.22 eV, respectively. Accordingly, at doping levels on the order of 1.times.10.sup.17 cm.sup.-3 and higher, these materials are highly sensitive to fluctuations in temperature since most of the dopants do not become electrically active until temperatures well above room temperature are reached. This property makes wide bandgap materials with deep impurities suitable for applications requiring a high degree of temperature sensitivity, such as thermistor applications, but less suitable for applications requiring temperature insensitive operation over a wide temperature range extending from room temperature to 1000 K and above.
However, because wide bandgap semiconductors such as diamond are expected to offer significant increases in figures of merit for electronic devices, compared to devices formed from silicon and other conventional semiconductors, attempts have been made to promote operation in the extrinsic region while inhibiting operation in the temperature sensitive ionization region. This is in contrast to the widely recognized use of wide bandgap semiconductors for thermistor applications, which require operation in the temperature sensitive ionization region.
One such attempt to improve the thermal stability of electronic devices fabricated from a wide bandgap semiconductor such as diamond is disclosed in an article entitled Active Electronic Applications for Diamond, by I. M. Buckley-Golder and A. T. Collins, Diamond and Related Materials, Vol. 1, pp. 1083-1101 (1992). In this article, the authors proposed relatively high doping levels in order to obtain greater concentrations of active carriers for temperatures below 1250 K and to reduce the activation energy (E.sub.a) of boron, the dopant impurity. Unfortunately, this leads to degraded device performance, by limiting the maximum depletion depth and preventing pinch-off in P-channel depletion-mode FETs and by limiting the hole mobility (.mu..sub.h). Moreover, in order to significantly reduce the activation energy of boron in diamond, doping levels on the order 1.times.10.sup.20 cm.sup.-3 are believed to be required. However, as described in a book entitled The Properties of Diamond, Academic Press, London, 1979, edited by J. E. Field, researchers A. T. Collins and E. C. Lightowlers disclose that these high levels of doping lead to undesirable parasitic conduction mechanisms such as impurity band conduction which can significantly degrade device performance.
Another attempt to use highly doped wide bandgap semiconductors such as diamond to achieve extrinsic operation over a relatively wide temperature range is disclosed in an article by K. Shenai et al. entitled Optimum Semiconductors for High-Power Electronics, IEEE Transactions on Electron Devices, Vol. 36, No. 9, September 1989, pp. 1811-1823. In particular, Shenai et al. propose that acceptor concentrations in diamond on the order of 2.times.10.sup.17 cm.sup.-3 can be used to compensate for an expected decrease in hole carrier mobility at high operating temperatures. Shenai et al. also proposes that conductivity enhancement at high operating temperatures can be achieved with the aforementioned high doping concentrations. However, the Shenai et al. work has been questioned for using low temperature approximations to describe the high temperature characteristics of diamond. In particular, in an article by A. T. Collins, entitled Diamond Electronic Devices--Can They Outperform Silicon or GaAs?, Materials Science and Engineering, B11 (1992), pp. 257-263, a high temperature approximation of free carrier concentration (p), based on an intentional acceptor concentration of 7.7.times.10.sup.17 cm.sup.-3, was substituted for the low temperature approximations of Shenai et al. Based on the high temperature approximation, the conductivity of P-type diamond was shown to vary over two orders of magnitude from room temperature to 1000 K and was also shown to be below the conductivity of GaAs at all temperatures.
U.S. Pat. No. 5,252,840 to Shiomi et al. discloses another approach to achieving high temperature stability for a diamond semiconductor device. More particularly, the patent discloses a high doped diamond layer, greater than 10.sup.20 cm.sup.-3 as a supplier of carriers and a undoped or low doped diamond layer as a conducting region of carriers and injecting the carriers from the high doped diamond into the non- or low-doped diamond by diffusion or an applied electric field. However, as noted in the patent, it is believed that no practical electronic device could be fabricated using only the non- or low-doped diamond layer.
Thus, notwithstanding the attempts to use wide bandgap semiconductors for operation over relatively wide temperature ranges, there continues to be need for a wide bandgap semiconductor device having relatively uniform characteristics extending over a relatively wide temperature range.